(a) Technical Field
The present disclosure relates to a method for manufacturing a hollow composite structure, particularly by air pressure without a tube. More particularly, it relates to a method for manufacturing a hollow composite structure by forming a prepreg product using a plurality of prepregs, forming a soft coating layer on an inner surface of the prepreg product, and compressing the prepreg product by applying pressurized air to the coating layer and, at the same time, applying heat to the prepregs.
(b) Background Art
Prepreg is a term for pre-impregnated fibers. A prepreg is typically a sheet-like product, in which reinforced fibers are pre-impregnated with a resin, i.e., binder, and is an intermediate product for producing a composite material product. As the reinforced fibers, carbon fibers, glass fibers, aramid fibers, etc. are mainly used. As the binder, epoxy resin, polyester resin, thermoplastic resin, etc. are typically used. A prepreg in the form of a fabric, such as cloth, can be molded into a predetermined shape by a mold, and cured to form a reinforced resin product such as a fiber-reinforced plastic (FRP).
The prepreg is formed into various products according to the type of fibers, the arrangement of fibers, and the type of binder used. The composite materials produced using the prepreg have improved properties such as strength, hardness, corrosion resistance, fatigue life, impact resistance, etc. compared to other materials.
In the automobile industry, a prepreg is mainly used in a vehicle body structure for weight reduction. For example, the prepreg is widely used in the bodies of Formula 1 racing cars, high performance homemade cars called supercars, etc. Further, prepregs are used in manufacturing vehicle interior materials, brackets, etc., and are also applied in various industries such as vehicles (including aircraft, boats, bicycles, etc.), golf clubs, high pressure containers, etc.
Examples of conventional methods for manufacturing hollow composite structures using a prepreg will be described below.
According to an autoclave molding method, a plurality of prepregs are stacked in a mold, the resulting mold is placed in a vacuum bag which is then sealed from the outside, a gas pressure is applied externally to the vacuum bag, a vacuum state is maintained in the vacuum bag by an external vacuum pump to remove volatile components generated from a resin impregnated in the product, thereby removing gaps between the respective prepregs to completely bond the prepregs.
According to a filament winding molding method, a fiber reinforced material is passed through a resin mixture and is continuously wound in the form of a filament on a cylindrical mandrel rotating in a wet state to be molded. This method is generally used to manufacture a hollow product such as a pipe, cylinder, etc. and can be used to manufacture products having various sizes such as a storage tank having a diameter of 6 m, motor having a diameter of 2.5 m, etc. In the filament winding molding method, it is necessary to properly control the way in which the filament moves, the winding angle, the left and right rotational movement of the mandrel, etc. Using a filament winding molding method, it is possible to manufacture products having a uniform thickness in mass production. After the fibers are molded, a model is molded using an autoclave or hot press.
According to a thermal expansion molding method, a prepreg is disposed in a mold and a silicon block also disposed in the mold is expanded at a temperature higher than the deformation temperature of the resin such that the prepreg is compressed. In particular, the prepreg is placed in a preheated mold having a predetermined shape, and is the prepreg is then compressed at high temperature and high pressure to produce a product. Using a thermal expansion molding method, it is possible to produce a product having a complicated shape within a short period of time.
According to an air compression molding method, an example of which is shown in FIG. 1, a sheet-like prepreg 6 is located in a mold 2 and heat is applied to the mold 2 using a heater. When heat at a predetermined temperature is applied to the mold 2, a tube 4 in the mold 2 expands to pressurize the prepreg 6.
In addition to the air compression molding method by heating shown in FIG. 1, the air may be pressurized to a tube to be expanded as shown in FIG. 2. In particular, as shown, a prepreg 5 is wound on a mandrel 7 in a rectangular tube shape, the resulting mandrel 7 is placed in a preheated mold 1, the mold 1 is closed such that the mandrel 7 (i.e., air tube) is expanded to pressurize the prepreg 5. When the prepreg 5 is completely cured, the mandrel 7 disposed therein is removed to complete the molding process.
For example, in Korean Patent Application No. 10-1994-0021050 describes one method for manufacturing a cylindrical composite material structure using a 2-piece mold, wherein a prepreg is stacked and fixed in a lower mold, pressurized by a nylon bladder that is expanded, and then cured.
According to Koran Patent Application No. 10-1992-0008038, a bicycle body formed using a composite material and a method for manufacturing the same is described. In particular, an air injector is inserted into a hole formed in a mold so as to penetrate a composite material layer, an airtight tube and a sealing member disposed in the tube supply compressed air thereto such that the composite material comes into close contact with an external mold, and the composite material is then cured.
According to Korean Patent Publication No. 10-2002-0016780, a hollow composite product and a method for manufacturing the same is described. In particular, a laminate is wound on a mandrel and placed in a preheated mold to be molded. After the mold is assembled, a pump is used to apply pressure to and expand a pocket disposed in the mold, thereby forming a rectangular composite tube.
According to U.S. Pat. No. 5,853,651, a high pressure hollow process for manufacturing composite structures is described. A mandrel made of thermoplastic material, such as polystyrene or acrylonitrile-butadiene-styrene co-polymer (ABS), having a laminate provided thereon is placed in a mold, and is heated and inflated to a predetermined pressure. When the laminate is completely cured, the mandrel disposed therein is removed to complete the molding process.
However, according to the above-described conventional air compression molding methods, the tube for compression molding of the prepreg is pre-formed into a shape that matches the frame of the mold, thus requiring a long manufacturing time which reduces productivity. Moreover, since the pre-formed tube can only be suitably used with a mold having a corresponding predetermined shape, use of the pre-formed tube is limited. Furthermore, to manufacture a plurality of hollow structures of various shapes, a variety of corresponding tubes for forming the various shapes must be prepared in advance. Further, when using a complicated mold, it is difficult to prepare the corresponding tube. Thus, manufacturing time and costs significantly increase, and competitiveness of the product is decreased. In addition, it is very difficult to apply a uniform pressure to all surfaces, such as corners and curves, of a complicated inner surface shape through use of the tube (as depicted, for example, in FIG. 3), and thus it is difficult to ensure uniform quality of the end product.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.